Currently, the above requirement is most common in the area of cellular communication systems. In such systems, the traffic handling capability has been shown to be improved by the inclusion of separate power amplifiers in the feed to each individual antenna array element, as opposed to the use of single higher powered amplifier device before beam formation. Further, in the interest of reducing the cost of base station maintenance, it has been proposed to site power amplifiers and diplexers at the base of the tower on which the antenna array is mounted. This normally involves lengthy cable runs of as much as 30 meters between each power amplifier and the antenna element with which it is associated.
It is essential, for preserving the desired beam shape, that each antenna element is supplied with the correct phase of signal so that the overall desired beam shape is preserved. This in turn means that there needs to be exact phase matching from the point of beam formation, through power amplification, diplexing and the up-feed cables to the individual antenna elements. However, this is very hard to achieve in the case where lengthy cable runs exist especially if the cable runs are exposed to different envoronmental conditions as individual amplifiers tend to be sensitive to signal amplitude, temperature and small independent variations. Also diplexers involve the use of high Q filter elements which are potentially susceptible to phase change. The problem is exacerbated by the fact that the individual amplifiers also produce phase changes such as exposure to heat from strong sunlight.
The problem of phase variations in a phase array antenna is known and various attempts have been made to solve the problem. U.S. Pat. No. 5,072,228 discloses an arrangement in which a phased array antenna has a digital phase shifter associated with each radiating element. The beam pattern is sensed using an integral monitor manifold which receives a part of the signal from all the radiating elements, produces a resultant signal which is then used as an input to a processor for calculating the phase error of a signal radiated from each element. The problem with this arrangement is that the changes due to each individual element are small as compared with the resultant signal from all the elements. This, in turn means that the arrangement does not produce a good measure of each individual element.
It is an object of the present invention to overcome the problems of the prior arrangements.